1. Field of the Invention
The present invention relates to a laser beam irradiating apparatus for annealing with a laser beam and, more specifically, to a laser beam irradiating apparatus used for forming a semiconductor substrate during manufacturing of semiconductor devices.
2. Description of the Background Art
A so-called SOI (Silicon On Insulator) structure in which a single crystal semiconductor layer is formed on an insulator is promising as a substrate material of VLSIs of the next generation. A method of melting and re-crystallization employing laser beam (laser anneal method) is one of the methods of manufacturing the SOI, in which a semiconductor layer of non-single crystal formed on an insulator is melted and re-solidified by laser beam irradiation so as to obtain a single crystal layer. Recently, laser anneal method has also attracted attention as a method of forming a polycrystalline semiconductor layer having large grain size on an insulator.
FIG. 8 is a block diagram showing one example of a conventional laser beam irradiating apparatus used for laser annealing. As shown in FIG. 8, the laser beam irradiating apparatus includes an argon ion laser source 1, shutters 2 and 3, a beam expander 4, a half-wave plate 5, a polarizing prism 6, total reflection mirrors 7 and 8, an X-axis rotation mirror 9, an f-.theta. lens 10, a reflecting mirror 11, a focus stage 12, a Y-axis stage 13, a susceptor 15 and a heater 16.
Argon ion laser source 1 generally has sufficient output to melt a non-single crystal layer and high output stability, and it is capable of continuous wave in the order of 20 W. Shutters 2 and 3 rotate to shut the path of the laser beam to stop laser beam irradiation. Beam expander 4 expands luminous flux of the laser beam emitted from argon ion laser source 1. Half-wave plate 5 receives the laser beam which has passed through beam expander 4 to change the inclination of the plane of polarization of the incident laser beam. Polarizing prism 6 receives the linearly polarized laser beam which has passed through half-wave plate 5, partially transmits the incident laser beam at a ratio corresponding to the inclination of the plane of polarization and reflects the remaining part of the laser beam in the orthogonal direction. Total reflection mirrors 7 and 8 reflect the laser beam which has been reflected in the direction orthogonal to the incident direction by polarizing prism 6, by a prescribed angle to guide the laser beam to a prescribed position. X-axis rotation mirror 9 changes the angle of reflection of the laser beam in the direction of the X-axis. F-.theta. lens 10 receives the laser beam which has been swung by X-axis rotation mirror 9, reduces the diameter of the laser beam such that the beam diameter becomes about 150 .mu.m.phi. on the surface of a silicon wafer 14 which is the object to be processed, and irradiates the laser beam while scanning the surface at a constant speed. Reflecting mirror 11 reflects the laser beam which has passed f-.theta. lens 15 and guides the same to the surface of the object 14 to be processed. Focus stage 12 holds X-axis rotation mirror 9 and f-.theta. lens 10, and adjusts focal length by changing the distance between f-.theta. lens 10 and mirror 11. Y-axis stage 13 holds X-axis rotation mirror 9, f-.theta. lens 10 and mirror 11, and it moves the position to be irradiated with laser beam on the surface of the object 14 in the Y direction, by moving these components in the Y direction. Susceptor 15 is for holding a silicon wafer 14. Heater 16 heats the silicon wafer 14 to improve efficiency of re-crystallization by the laser beam.
The output stability of continuous wave argon ion laser source 1 is, at present, about .+-.0.5%, which is satisfactory when uniformity of the laser power required in laser annealing is taken into consideration.
The laser beam passes through optical components such as a plurality of mirrors 7, 8, 9 and 11, lenses 4 and 10 and so on and directed to the surface of a sample (silicon wafer 14) held by susceptor 15. In this example, the power of irradiating laser is adjusted by half-wave plate 5 and polarizing prism 6. Scanning of laser beam is carried out by X-axis rotation mirror 9 in the X direction, and by movement of Y-axis stage 13 in the Y direction.
The laser beam has its diameter reduced to about 150 .mu.m.phi. through f-.theta. lens 10 and irradiates the surface of the sample. Though it depends on the laser output and speed of scanning of the laser beam, a strip-shaped area extending in the X direction having the width of about several ten to several hundred .mu.m is re-crystallized by one laser beam scanning in the X direction. Therefore, re-crystallization of the entire surface of the sample is achieved by repeating a procedure in which laser beam scanning in the X direction is done, the laser beam is moved in the Y direction such that the area re-crystallized by the last laser beam scanning in the X direction is partially overlapped with the area to be re-crystallized, and again the laser beam scanning in the X direction is carried out.
However, when laser annealing is effected by the conventional laser beam irradiating apparatus, the entire surface of the sample can not be uniformly re-crystallized, as will be described hereinafter. Specifically, the area which is re-crystallized by one laser beam scanning in the X direction and in the Y direction differs from the width of the area re-crystallized by another laser beam scanning.
It has been considered that this difference or variation of the width of the area which is re-crystallized stems from a power loss of the laser beam. Thus laser beam power was measured by means of a calorimeter. However, the laser beam power did not change at all, and the cause of variation could not be found.
Presently, materials of optical components used in such a laser beam irradiating apparatus are selected on the basis that the optical components are not damaged by laser beam irradiation, and that power loss of the laser beam is small. Based upon such considerations, optical components formed of an optical glass, such as BK7, are used without any problem for an argon ion laser of the order of 20 W.
The inventors have discovered that the source of the problem of variation of the width of the area which is re-crystallized does not stem from a power loss of the laser beam, but caused by fluctuation of laser beam power distribution caused by thermal deformation with age of the optical components.
FIG. 9 shows a result of measurement of laser beam power distribution in the direction of the diameter of a laser beam spot when a pin hole is moved in the direction of the diameter of the beam spot and the laser beam transmitted through the pin hole is measured by measuring equipment, in the conventional optical system using the optical material BK7. As shown in FIG. 9, power distribution of the laser beam (hereinafter referred as beam profile) was measured and it was found that the beam profile rapidly changes as time passes from the start of laser beam irradiation. In FIG. 9, the abscissa corresponds to the position in the diametrical direction of the laser beam spot, while the ordinate corresponds to the laser power per a prescribed unit area. FIG. 9 shows results of several measurements carried out at prescribed intervals while continuously irradiating the laser beam to find variation of the beam profile. It is considered that variation of the beam profile is due to thermal deformation of optical components as the components were locally heated by the laser beam irradiation.
FIG. 10 shows a state of re-crystallized areas on the surface of a sample which has been subjected to laser annealing by a laser beam irradiating apparatus employing a half-wave plate 5 and a polarizing prism 6, of which base material is optical glass BK7. Referring to the figure, hatched portions represent re-crystallized areas. The laser beam scans in the X direction and the scanning is successively repeated in the Y direction. As is apparent from this figure, the width of the area re-crystallized by one scanning decreases as the scanning proceeds in the Y direction. As stated previously, by the conventional method of evaluation in which laser beam power is measured by a calorimeter, there is no change found in the laser beam power. However, as shown in FIG. 9, only the area irradiated by the laser beam having power higher than a prescribed level W1 of the laser beam spot is re-crystallized. Therefore, if the beam profile changes and the power at the central portion of the spot decreases, the area which can be re-crystallized becomes narrower. This is the reason why the width of the area which is re-crystallized varies even though the laser beam power itself does not vary. If the laser beam having sufficiently larger power than necessary for re-crystallization is irradiated, there is not a possibility of failure in re-crystallization. However, the width of the area which is re-crystallized still changes.